Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 08/09/2018 09:33:09

Title: Does a Crooke's Radiometer behave differently in an ultrahigh vacuum?
Post by: chris on 08/09/2018 09:33:09

I was speaking with a light person over lunch the other day and we got to talking about photon momentum (which he works on), the YORP (Yarkovsky–O'Keefe–Radzievskii–Paddack) Effect and, off the back of that, Crooke's radiometer.

(https://upload.wikimedia.org/wikipedia/commons/1/1d/Crookes_radiometer.jpg)

We were debating the workings of the radiometer and my friend said he thought the radiometer would behave differently in an ultrahigh vacuum compared with the "standard" vacuum traditionally used. He went as far as to suggest that it should rotate the other way.

Is he correct?

Title: Re: Does a Crooke's Radiometer behave differently in an ultrahigh vacuum?
Post by: Bored chemist on 08/09/2018 13:12:28

Quote from: chris on 08/09/2018 09:33:09

Is he correct?

Yes.

Title: Re: Does a Crooke's Radiometer behave differently in an ultrahigh vacuum?
Post by: yor_on on 08/09/2018 17:51:05

Are you sure?
https://golem.ph.utexas.edu/category/2008/07/light_mills.html

(couldn't help myself after reading the comments under the article :)

Title: Re: Does a Crooke's Radiometer behave differently in an ultrahigh vacuum?
Post by: alancalverd on 08/09/2018 18:04:13

The standard Crookes radiometer works by photon absorption heating the black surfaces, which then transfer energy to residual gas molecules that fly off the surface. Conservation of momentum propels the black surface backwards - say clockwise. This is observed.

In an ultrahigh vacuum, photons are absorbed by the black surface and reflected from the mirrors. The mometum of a photon of energy E  is E/c, so if the reflected photon has the same wavelength as the incident photon (perfect mirror) the interaction must have transferred 2E/c to the mirror whilst a photon of the same energy absorbed by the black surface will only transfer E/c in the opposite direction. Thus in principle the same vane will rotate anticlockwise if the vacuum is "hard" enough. Not sure if this has ever been observed in a laboratory instrument.

You can cheat by applying a brief , intense pulse of radiation to the "mirror" side of the vanes only, using a laser or focussed light bulb. this will produce temporary asymmetric heating that drives the vanes anticlockwise in a soft vacuum, but the process is less efficient than if he pulse is applied to the black side of the vanes.

Title: Re: Does a Crooke's Radiometer behave differently in an ultrahigh vacuum?
Post by: Bored chemist on 08/09/2018 18:06:34

Quote from: yor_on on 08/09/2018 17:51:05

Are you sure?

Yes, if the light is bright enough.

There/s more detail than most people want to know about it here
https://en.wikipedia.org/wiki/Crookes_radiometer#General_description

Title: Re: Does a Crooke's Radiometer behave differently in an ultrahigh vacuum?
Post by: chris on 08/09/2018 18:10:40

Quote from: alancalverd on 08/09/2018 18:04:13

Not sure if this has ever been observed in a laboratory instrument.

This was going to be my next question. Has this been demonstrated experimentally?

Title: Re: Does a Crooke's Radiometer behave differently in an ultrahigh vacuum?
Post by: yor_on on 08/09/2018 18:14:10

It's correct that it turns around in a cold environment, just as you said BC,, and space definitely is a cool place. But reading the comments I still don't know how it works, well there are some pretty shrewd ideas in the link but? :) Check the link I gave out and see if you guys make more sense of it. And yes, the link contains both simulations you can try and practical tests.

Title: Re: Does a Crooke's Radiometer behave differently in an ultrahigh vacuum?
Post by: yor_on on 08/09/2018 18:33:31

And thank you Chris for bringing this up, it made my day.